A number of processes are known for depositing or removing thin film material onto or from a semiconductor substrate. Typically, these processes occur in a reaction (etch or deposition) chamber containing a highly reactive environment such as produced in a chemical vapor deposition (CVD) reaction between a carrier gas, such as hydrogen and a precursor, such as trimethylamine alane, or nickel pentamethylcyclopentadienyl allyl, among others, producing a solid reaction product, such as aluminum and nickel, respectively.
The resulting solid reaction product, however, produces a residue which is not only deposited onto the substrate, but on the interior of the walls of the reaction chamber, as well.
Over time, and in the course of an accumulated number of coating runs, sufficient buildup occurs produce a layer which, if unchecked, will eventually peel from the walls, and fall onto the substrate. Given the stringent specifications (on the order of 1.times.E-3 Angstroms) for the avoidance of particles, such peeling could produce a deleterious effect.
Therefore, after a certain number of coating or etching runs, processing must be interrupted in order to clean the chamber so as to remove the deposited wall residue. Presently, empirical data is used to determine the number of coating runs which can be performed prior to a necessity to clean the chamber walls. This data, however, is highly process dependent and can easily be influenced by extraneous factors (e.g., ambient atmosphere, humidity) or a change in processing conditions. There is no current detection apparatus available for determining when the interior walls of a deposition chamber have reached a threshold thickness value.
Moreover, it has been observed that the time required to clean the chamber does not vary linearly, but varies exponentially based on the thickness of the accumulated wall residue thickness. Time is an important factor because the variation in cleaning times (3-4 hours versus 15-18 hours) can greatly increase chamber down time. There is a perceived tradeoff to put off cleaning to an intermediate time, or put differently, a wall thickness threshold, to satisfy the competing concerns to optimize cleaning and down time most efficiently. In addition, it is frequently advantageous to leave a small amount of built up wall residue to avoid changing the surface of the chamber with time as would occur if all of the material were to be removed. To date, no means have been developed for monitoring the wall thickness on a real-time basis without resorting to empirical data, or by interrupting the process by opening the chamber.
It has been found that quartz-crystal sensors, such as these manufactured by Leybold Inficon, Inc., of East Syracuse, N.Y., are capable of the high precision resolution, reliability, and accuracy required to solve the above problem and to monitor wall thickness variation in a repeatable manner. However, a system incorporating these types of sensors has not been readily feasible due to concerns relating to the corrosive and highly reactive environment found within an etch or deposition chamber, as well as competing concerns which are electrical and thermal in nature.